(1) Field of the Invention
The present invention relates to a CRT drive circuit for driving a cathode ray tube (CRT).
(2) Description of the Prior Art
FIG. 9 is a circuit diagram showing a typical CRT drive circuit. Referring to the same Figure, reference numeral 1 designates a video amplifier for amplifying a video signal, numeral 2 designates a cut-off circuit for adjusting the amplified video signal and supplying it to a cathode. Numeral 3 designates a fly-back transformer for supplying a high voltage to an anode and a focus voltage to a focus electrode, numeral 3a designates a primary winding of the fly-back transformer, numeral 3b designates a secondary winding of the fly-back transformer, numeral 4 designates a screen voltage generating circuit for supplying a screen voltage to a screen electrode, numeral 4a designates an operational amplifier, and numeral 4b designates a variable resistor for supplying a set voltage to the noninverting input side of the operational amplifier 4a. Numeral 5 designates a horizontal output element comprising transistors, numeral 6 designates a diode, numeral 7 designates a capacitor, numeral 8 designates a contrast/brightness variable resistor having a contrast variable resistor 8a and a brightness variable resistor 8b which are operated by users.
Next, the operation of the CRT drive circuit shown in FIG. 9 will be described. As shown in the same Figure, a video voltage amplified by the video amplifier 1 and then adjusted by the cut-off control circuit 2 is applied to the CRT cathode. A high voltage and a focus voltage are applied from the fly-back transformer 3 to the CRT anode and the focus electrode. Further, a screen voltage is applied from the screen voltage generating circuit 4 to the CRT screen electrode.
When a pulse is applied to the horizontal output element 5, a high voltage pulse is generated by means of the diode 6, the capacitor 7 and the primary winding of the fly-back transformer 3, so that the high voltage and the focus voltage are applied to the CRT anode and to the focus electrode by means of the fly-back transformer 3. A screen voltage is branched from the primary winding 3a of the fly-back transformer 3 through rectifying to be applied to the CRT screen electrode according to a set voltage applied to the operational amplifier 4a in the screen voltage generating circuit 4 by rectifying the aforementioned generated high voltage pulse.
Focus adjustment of the CRT is achieved by setting the screen voltage so that focus is optimal by preliminarily adjusting the variable resistor 4b and further adjusting focus so as to be optimal by adjusting the focus voltage. Further, the brightness of the CRT may be adjusted by adjusting the DC voltage by means of the contrast/brightness variable resistor for users and then supplying the DC voltage to the video amplifier 1 and the cut-off control circuit 2 respectively.
Meanwhile, this kind of the CRT drive circuit is used for CRTs having a plurality of resolutions. For example, as a CRT for a plurality of resolutions in accordance with the specification of mask pitch is used.
Generally, when the CRT drive circuit is applied to a CRT having a plurality of resolutions, if the screen voltage is set by giving preference to focus so that characters displayed on a display arc clear, raster moire may appear depending on resolution so that the display becomes unclear.
Conventionally, the variable resistor 4b has been preliminarily adjusted so as to keep the screen voltage always low in order to suppress influence of the screen due to such a resolution gained when the raster moire is generated. However, if such a constant low screen voltage is set, the focus deteriorates alone unnecessarily on screen with other resolutions in which no raster moire is present, so that displayed characters and the like blur.
To cope with raster moire which occurs under any resolution due to mask pitch as described above, conventionally it has been necessary to reduce the screen voltage with the sacrifice of focus. Thus, with conventional manner, there has been such a problem that focus may deteriorate on displays with other resolutions in which no raster moire is present.